1. Field of the Invention
A present invention relates to a signal transmission system for performing one-to-many internode data transmission using a serial transmission path.
2. Description of the Related Art
In recent years, the data processing rate of LSIs has increased dramatically with the improvement of semiconductor integration technology. With the increase in data processing rate, wiring substrates for mounting semiconductor integrated circuits thereon are required to improve their signal transmission capacity. Particularly, even in personal computers, which are high-ranking server-type systems, a so-called parallel processing architecture is recently adopted, in which a plurality of high-speed CPU chips are provided. For example, the technology of parallel processing architecture is disclosed in “Parallel Computer” Hideharu Amano, pp.6-13, SHOKODOCO., LTD. According to this, when a system including a plurality of modules such as CPUs for performing data processing is constructed, the method for connecting the modules are classified into a bus connection type, a switch connection type and an interconnection network type. Of these types, the bus connection type is not suitable for connecting a large number of modules, but there is a merit that it has a simple structure in comparison with the other types, while the quantity of hardware is small and the scalability is also excellent. Thus, the bus connection type is often used in commercial computers including personal computers, or computer application products such as page printers.
For mounting intermodule connection portions in a parallel processing system, a large number of connectors and wires are required. In order to improve the communication performance or the wiring density, the wires are therefore formed as multilayer wires or micro-wires. However, such a method is reaching a limit due to signal delay or transmitted waveform distortion caused by inter-wire capacitance or connection wire resistance. In addition, with an increase in working speed, electromagnetic interference (EMI) also becomes a main problem.
In such a manner, the throughput of data processing apparatus has been often restricted by the transmission capacity of a bus on a wiring substrate. In order to break down such a limit of an electric type bus, it comes under consideration to use an intrasystem optical connection technique, which is called “optical interconnection”. Various forms have been proposed in accordance with the details of the system configuration as disclosed in “The 9th JIPC Annual Meeting” Teiji Uchida, 15C01, pp.201-202, “Packaging Technology for Optical Interconnects” H. Tomimura, et al, IEEE Tokyo, No. 33, pp.81-86, 1994, and “Electronics” April in 1993, Osamu Wada, pp.52-55, where the summary of the optical interconnection technology is described. According to the optical interconnection technology, there are merits that EMI can be reduced in spite of working at a frequency that can be higher than in the electric type, that no physical connection wire is required for bus signal lines, that the transmission bandwidth can be expanded by multiplexing using wavelength, intensity or the like, and that simultaneous two-way communication can be attained.
Particularly, spatial optical transmission technology is in good consistency with the bus connection type parallel processing architecture because it can attain simultaneous communication among many ports differently from the transmission technology using optical fibers. A related technology is disclosed in JP-A-10-123350. This technology attains optical communication among ports placed in end surfaces of a flat-plate type light guide path, in which incident signal light from one end surface is diffused and transmitted to the opposite end surface so as to attain broadcast communication. By use of multiplex transmission, a plurality of broadcast communications independent of one another can be carried out simultaneously.
As such a signal transmission system technique making use of spatial optical transmission, there has been proposed a system in which a serial signal (serial data signal) obtained by parallel-to-serial conversion of a plurality of electric signal lines is optically transmitted through a spatial optical transmission medium in order to relieve the problem of transmission skew among signals and to reduce the number of transmission paths. Particularly, here is adopted a system in which only a data signal and a frame signal are transmitted from a transmitting node to a receiving node without transmitting a clock signal. The frame signal means a transmission frequency of an electric signal line, while the clock signal means a transmission frequency of the serial signal obtained after the parallel-to-serial conversion. The clock signal is required for latching the serial signal in the receiving node. Here, the clock signal is generated automatically by a unit such as a PLL (Phase Locked Loop) circuit multiplying the frequency of the received frame signal. As a result, it is not necessary to transmit the clock signal having a high frequency particularly in optical transmission. Thus, the transmission band required by optical components and circuit components constituting a transmission path can be narrowed so that the transmission system can be constructed easily.
Such a system for transmitting a serial data signal obtained by parallel-to-serial conversion uses a technique for converting a parallel data signal into a serial data signal by parallel-to-serial conversion, transmitting the serial data signal, receiving the serial data signal, and then restoring the received serial data signal to the original parallel data signal by serial-to-parallel conversion. In this case, even if the serial data signal can be latched in a reception circuit, the bit order will be shifted from that of the original parallel data signal when the transmission delay time of the synchronization signal differs from that of the serial data signal. Therefore, processing for shifting bits in position is typically performed by an alignment circuit in the reception circuit so that the signal is aligned to be restored to its original bit order. This processing can be applied to one-to-one or one-to-many internode signal transmission.
However, when signal is transmitted from the multi-node side to the single-node side in the one-to-many internode signal transmission, one and the same reception node receives signals from different transmission nodes. Therefore, the quantity with which the bit positions should be shifted differs from one transmission node to another. There can be therefore considered a method in which a transmission node number is identified in the reception node, and a correct shift quantity is changed for each transmission node. However, this method has such problems that the circuit becomes complicated, and the method cannot be used when the reception node cannot identify a transmission node.